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Power over Ethernet
Updated: July 22nd, 2005
Rev: 2.0
White Paper - Axis Communications
Power over Ethernet
1 Introduction
Power over Ethernet (PoE) is a convenient and secure way to provide power to devices such as
network cameras, IP telephones and wireless device points. By injecting power into the network
cable, the requirement for local power at the location of a device is removed. And this is done
without any degradation of network performance. There are a couple of clear benefits associated
with the use of PoE:
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•
•
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Reduced cabling, only one cable is required
Easy installation for locations which are hard to reach, as no local power is needed
No need for certified electricians
A centrally provided power provides an easy way to add UPS functionality
Improved security, no risk for power interruptions due to “borrowed” power outlets
Lower system costs since new power outlets do not need to be installed
A PoE system contains of two basic components:
•
Power Supply Equipment (PSE). This is a unit, which inject power into the network
cable. There are two types of PSE available:
o
o
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Midspan. This is a standalone injector which just adds power to a network
cable. It is used when adding PoE functionality to an existing network.
Normally available in 1, 6, 12 or 24-port versions.
Endspan. This refers to network switches that have built-in PoE functionality in
addition to its switching functionality.
Powered Device (PD). This is the device being powered via the network. It can either
have built-in functionality for PoE, such as the AXIS 221 network camera, or use an
external splitter such as the Power over LAN Active Splitter offered by Axis. The
external splitter is a small unit that splits network and power into two separate
functions/cables, and is used to provide PoE functionality to products without built-in
support for PoE.
Figure 1. Example of a PoE-based system
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White Paper - Axis Communications
Power over Ethernet
2 How does it work
2.1
Basic description
All networked products have two things in common; they require a network connection and a
power supply. Networking is normally achieved by providing network sockets at the location of
the networked product connecting to the facilities network infrastructure. The power supply
normally means using the nearest power socket without considering what other devices are
connected to this group or knowing where the fuse for this group is located. If controlled power
is needed, e.g using UPS, it normally means providing separate power cabling from one or two
locations within the facility.
Using PoE combines these two functions into one cable, the network cable.
2.2
The standard
The functionality of PoE is defined in the IEEE 802.3af standard, and it describes the power
requirements as well as detecting and removing devices.
The standard covers two different means to carry the power. The first one is built around using
the spare wires (4,5,7 & 8) in a Cat 5 or 5e cable, while the other one uses the data wires (1,2,3
& 6) and so called Phantom feeding which means that power and data will share the same
physical wires. The benefit of using Phantom feeding is that functionally is present when power
is and the other way around, so a bad connection or damaged network wire will not cause the
product to be powered up without being correctly networked which may be the case when
different wires are used. The standard defines that all PDs must comply with both methods of
providing power.
The standard provides 48VDC and a maximum of 15.4W per port. Due to expected cable losses,
a PD may require no more than 12.95W
The operating distance for a PoE connection is the same as for a normal network connection
using Cat 5 or 5e cabling, i.e. up to 100m (328 ft).
To improve the performance of a PSE, the unit will classify connected devices based on its
power requirement. Classifying a PD is optional, if the device does not support this, it will
automatically be classified as Class 0.
2.3
Connecting a device
Connecting a 48V supply to network devices such as laptops would probably do permanent
damage to the unit, so a PSE does not apply power to a port until the connected device has been
detected and verified as a PoE enabled device. This is all done automatically upon connecting a
cable to the port.
The PSE does this using a technology called Resistive Power Discovery, which means that the
unit sends out two different short low voltage discovery signals (between 2.7V and 10.1V), with
at least 2 ms in-between, to the PD.
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White Paper - Axis Communications
Power over Ethernet
Once this detection is done, and the device is verified as PoE enabled, the unit will try to verify
the classification of the PD. A PoE device is classified in one of 4 different classes, depending
on the power consumption of the unit. This helps the PSE to optimize its performance.
Class Power Requirement Comment
0
0.44 – 12.95W
Default class
1
0.44 – 3.84W
Optional
2
3.84 – 6.49W
Optional
3
6.49 – 12.95W
Optional
4
For future use
For future use
Table 1. Available PoE classifications
Once the device has been verified and classified, the unit will provide full power to the port.
It is worth noting that most multi-port PSEs are not capable of providing full power to all ports
simultaneously. It has a certain “power budget” that it will distribute to the ports, depending on
the need of the PD’s. If the power required by all connected PDs exceeds what the unit can
provide, the unit will start to disconnect devices one by one, until power requirements are below
the unit’s maximum capacity.
2.4
Disconnecting a device
Just as with the connection of a device to a PSE, the removal of the device is a controlled action.
The PSE will constantly monitor the device and if it is removed, the unit will disconnect the
power within 300-400ms. This is done to avoid new items, installed right after the removal of a
device, from being damaged.
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Power over Ethernet
3 Installing IP-surveillance using PoE
As image quality and frame rate from network cameras and video servers have now surpassed
the performance of analog cameras, and high speed networks are commonly available at
reasonable cost, IP-surveillance systems based on streaming digital video over Ethernet are now
replacing the functionality of analog systems. And with the capability to power up devices using
the same network cable, there are additional benefits and the installation is more cost efficient.
Today, IP Surveillance systems are being installed in many different environments, some of the
more common once are:
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Education
Retail
Transportation (such as airports terminals)
Government (such as prisons)
Banking and Finance
There are differences in the requirements for the Surveillance system from each type of
environment. A train station installation is very different from a warehouse installation, and this
has little in common with a prison installation. Fortunately, most network device installations
share a similar infrastructure design.
Ethernet cables run from the network switch, through a patch panel, out of the communication
room and terminate in network sockets located throughout the facility. Devices such as a
network camera, IP telephones, print servers and computers are then connected to these sockets
using a short connection cable (See Figure 2). Adding PoE enables all PoE-ready devices such
as network cameras, to be powered through the same cabling infrastructure, providing the most
cost effective and secure solution.
If the switch is already installed, the simplest means to add PoE is by adding a dedicated
midspan. In new installations, the most convenient and cost effective solution would be to add a
switch with built-in PoE functionality, a so-called endspan or PoE-enabled network switcher,
which are available from manufacturers such as Cisco, 3Com, HP and Netgear. Smaller SoHo
oriented switches with PoE functionality have become more freely available.
Figure 2: Network Video system Architecture Using Power over Ethernet
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White Paper - Axis Communications
3.1
Power over Ethernet
Added values using Power over Ethernet
Some of the key benefits for PoE within the Security market are:
Easy to change camera position
Altering a camera position no longer requires installation of a new power outlet. It is possible to
easily experiment with different camera positions to achieve ultimate coverage results.
UPS backup for the security network
By using UPS backup for the PSE/network switch in the server room, the entire camera network
can continue operation during a power outage.
Add secure intrusion functionality
If the network cameras alarm input is connected to an alarm device such as a door sensor or
intrusion detector, the fact that the camera is powered from a central location, most likely with a
UPS back-up, provides the user with not only video, but also intrusion detection and other alarm
functions over the same monitored and centrally powered network.
3.2
Limitations with PoE
PoE presents many benefits, but comes with some limitations that are important to be aware or.
Outdoor installations
When installing cameras in outdoor environment, the camera must normally be protected
against the environment by having a protective housing. This housing is normally equipped with
either a heater to remove condensation from the front glass of the housing or a fan to circulate
air inside the housing to avoid overheating. And both these devices require power, which cannot
be provided by the PSE. But PoE may power the camera itself, as long as the additional
heater/fan is powered locally. The benefit of this is that the camera is powered from a controlled
central location, and may be monitored. The camera would not be affected by a local power
failure.
PTZ-cameras
A dome camera or one with Pan-Tilt-Zoom functionality will require additional power for the
motors and the power specification for most cameras with PTZ-functionality is above what can
be provided by PoE. There are non-standard PoE-systems, capable of providing higher power,
but these are manufacturer specific. Work is ongoing to provide a new standard for up to 30W,
but this is not yet finalized.
Wireless cameras
Due to the nature of these cameras, they have to be powered locally. But there are several access
points on the market with embedded PD functionality. This makes it easier to install the access
point where one can receive the best signal coverage, without having to consider availability of
power. So PoE also has benefits for wireless network cameras.
Use of additional add-on products
Some network cameras have the ability to power third party devices with limited power through
separate connectors on the camera. This functionality is only available if the cameras is powered
using the supplied power supply, and not when being powered using PoE. In some cases, the
camera may consume more power that its actual classification allows, which may cause the
midspan/endspan to turn of the port due to “power overload”.
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White Paper - Axis Communications
Power over Ethernet
Limited length
Just as with a normal network cable, the maximum distance from the switch/midspan to the
device is 100m (328 feet). Systems may operate over longer distances, but functionality and
performance cannot be guaranteed.
3.3
Design considerations
Using PoE is straightforward. Normal network design considerations also apply to IPsurveillance systems, but there is also the need for network sockets at “odd” locations, such as
in the ceiling and other areas not normally networked. In most cases, the natural location for a
camera would be fairly high up in a corner, either on the wall or directly in the ceiling, as this
normally provides the best images over the scene to be monitored and, at the same time, also
reduces the ability to cover the camera, realign it to face the ceiling or even steal it. Also
network cables and network sockets should preferably be concealed or located out-of-reach to
avoid disconnection/manipulation.
Spare capacity
When designing a PoE-based IP-surveillance system one should, as with all systems, design it
so that a spare capacity of at least 15-20% is available for expansion. Populating all ports on a
PSE from the start, would limit the ability to quickly add a camera, or to swap ports when
troubleshooting. Leaving at least one or 2 ports unpopulated on PSEs would provide the
required flexibility.
As most PSEs cannot provide full power on all ports simultaneously, it is also important to
ensure that the maximum power capacity of the PSE is not exceeded.
Don’t take short-cuts
It may be tempting to use some AC outlets that are available, to save some equipment costs, but
this has the following implications:
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The “vacuum cleaner” effect – cleaning personnel unplugging cameras, to use an
existing AC outlet, as they are easy to find, creating coverage breaks in the security.
“Where’s the fuse?” – locating the correct fuse to a power outlet in a larger building
may take some time, and using the normal power supply is always a risk, as one can
never know what other devices may be connected, such as powerful work tools etc.
Maintaining UPS capability. This creates a back up capability of the entire IPsurveillance system by backing up the PoE midspan or endspan.
Install in a secure environment
Install all midspans and endspans in locked and secure communications rooms. To minimize
tampering with the units, they should preferably be rack mounted.
Use colour cabling
Use a consistent colour coding for powered camera cabling, especially within patch panels, to
indicate that these cables are not to be touched, being part of the security system and including
power.
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White Paper - Axis Communications
Power over Ethernet
4 Glossary
The following words are normally used in conjunction with PoE
802.3af
Definition of the standard for Power over Ethernet – IEEE 802.3af
PoE
Power over Ethernet
PoL
Power over LAN, commonly used name for non IEEE 802.3af compatible
Power over Local Area Networks.
PSE
Power Sourcing Equipment. Equipment providing power to devices. May be a
dedicated standalone power injector or integrated into a network switch.
PD
Powered Device. Equipment being powered via the network, such as network
cameras, IP telephones and wireless access points.
Endspan
Another name for a switch with integrated PSE functionality.
Midspan
Separate PSE device, supplying power in networks with existing switchers.
UPS
Uninterruptible Power Supply. A unit providing backup power supply for
equipment when there is a power failure. By using built-in batteries, the unit
will provide power to mission critical devices for a “limited” time.
5 Conclusion
This white paper has explained the value of and the technology behind PoE. Whether you
decide to install a new network using PoE-ready switches, or you prefer to continue using an
existing infrastructure adding PoE functionality in the form of midspans, you will still make an
investment in well established and standardized technology available from all leading
manufactures of network products. This will provide you with a good foundation for your
security system.
PoE for the security market provides security managers the simplest, safest and most costeffective solution for networking and powering network cameras in a secure and controlled
way. The advanced features of PoE also vastly simplify the on-going maintenance of the
security network, enabling reliable, continuous operation with minimum downtime.
6 About Axis
Axis increases the value of network solutions. The company is an innovative market leader in
network video and print servers. Axis products and solutions are focused on applications such as
security surveillance, remote monitoring and document management. The products are based on
in-house developed chip technology, which is also sold to third parties.
Axis was founded in 1984 and is listed on the O-list (Attract-40) of Stockholmsbörsen
(XSSE:AXIS). Axis operates globally with offices in 16 countries and in cooperation with
distributors, system integrators and OEM partners in 70 countries. Markets outside Sweden
account for more than 95 % of sales.
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White Paper - Axis Communications
Power over Ethernet
Information about Axis can be found at: www.axis.com
Head office
Axis Communications AB
Emdalavägen 14
SE-223 69 Lund, Sweden
Tel: +46 46 272 18 00
Fax: +46 46 13 61 30
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